Medical infusion pumps are used in a variety of medical therapies and treatments to administer drugs, nutritional compositions, and prescribed fluids or fluid-like substances (collectively, “medicaments” or “infusates”) to patients in volume- and time-controlled doses. Medicaments can be accurately and continuously administered by such pumps, at infusion rates typically ranging from as low as 0.1 ml/hr to as much as 1200 ml/hr. Because of their ability to deliver medicaments in a precise, accurate, and continuous manner, the types of medical infusion pumps commonly referred to as syringe pumps are well-suited to pain management and palliative care, for instance, and are also often useful in neonatal and pediatric intensive care units.
In operation, a medicament syringe includes a plunger that slideably, but tightly, fits inside of a cylindrical barrel which in turn contains a medicament. Medicament is administered from the cylindrical barrel by shifting the plunger through the cylindrical barrel. Medicament is thus expelled from the cylindrical barrel under pressure, through a delivery route comprising a needle, nozzle, tubing, or the like, directing the medicament to a patient. When a syringe is installed in a syringe pump, the plunger of the syringe in the pump can be acted upon by a plunger driver of the pump that functions to controllably expel medicament from the syringe for delivery to the patient as aforementioned. Medicament from a syringe in a syringe pump can be administered, for example, intravenously, intra-arterially, epidurally or subcutaneously. As will be appreciated, a constant and metered application of medicament requires a steady and accurate flow from the syringe in the syringe pump through the delivery route or path to the patient.
During the course of delivering the medicament to the patient, it is possible for an occlusion to arise in the delivery path. Examples of occlusions can include a closed stopcock, slider valve or pinched line or tubing leading to the patient. Such a condition, if undetected, can cause injury to the patient. That is, when an occlusion occurs along the delivery path, medicament may not be delivered to the patient even though the syringe pump apparently continues to function. Thus, an occlusion prevents the syringe pump from delivering medicament to the patient until the occlusion can be detected and cleared from the infusion path. For this reason, the rapid detection of occlusions along the delivery path is important to reliable syringe pump operation.
An occlusion in the infusion line can cause a force, or pressure, in the syringe to increase. In turn, force between the plunger driver of the syringe pump and the plunger can increase. A force sensor can be placed in communication with the plunger driver, as shown, for example, in U.S. Pat. Nos. 7,635,349, 6,551,277, and 8,876,793, and in U.S. Patent App. Publ. No. 2015/0133890, each of which are incorporated by reference herein. A build up of pressure beyond normative operating ranges can accordingly be sensed, triggering alarms and initiating corrective action.
Many syringe pumps are capable of accommodating a range of syringe diameters or sizes (e.g., 1 ml through 60 ml capacities). Such a pump is disclosed, for example in the aforementioned U.S. Patent App. Publ. No. 2015/0133890 (U.S. application Ser. No. 14/400,509) that is incorporated by reference herein. But in many syringe pumps, generally the plunger driver and force sensor can experience varying occlusion force vectors depending upon which particular size of syringe is being used in the syringe pump, leading to varying accuracy and responsiveness overall in the pump's occlusion sensing system. One source of inaccuracy arises from misalignment of the force sensor with an axis of the plunger in the syringe. In particular, the force sensor may be aligned with the plunger in some syringe diameters or sizes when installed in the pump; but such desirable alignment of the force sensor with the plunger of the syringe might not occur when a syringe of another diameter or size is installed in the pump. Consequently, providing axial alignment of the force sensor with the axis of the plunger, or compensating for misalignment thereof, can provide improvement in the accuracy of force measurement.
A syringe pump that is capable of aligning the force sensor with the axis of a plunger of a syringe installed in the pump, across a range of syringe diameters or sizes, would therefore provide a distinct advantage in minimizing force measurement inaccuracy.